So-called radiopharmaceuticals or radiotracers, which are substances marked with a radionuclide, are used for imaging in the field of positron emission tomography. These radiotracers are usually administered to the patient by means of an injection, or possibly using another method. Positrons are emitted when the radionuclides decay and respectively interact with one electron. These two particles annihilate as a result of this interaction, so that high-energy photons are created which can be detected as annihilation radiation.
F-18-fluoro-2-deoxyglucose (F-18-FDG) is a typical radiotracer used in positron emission tomography. Positron emission tomography studies (PET-studies) using this tracer and different radiotracers are used for example to monitor therapy progress in the case of gastrointestinal stromal tumors (GIST) inter alia, by way of example after therapy using Imatinib. Therapy monitoring by means of PET permits early detection of possible therapy failures or insufficient dosage by precisely quantifying the metabolism as it progresses.
However, it is unclear whether (possibly determined) reduced uptake of a radiotracer in a tissue or body region is due to a metabolic change during the progress of the therapy, or whether it is due to changed receptor expression in a pathology and/or whether hypoperfusion is present. Such uncertainty exists mainly in therapies combined with anti-angiogenesis (for example, in therapy regimes whose starting point is the vascular endothelial growth factor (VEGF) signal molecule), but also in other therapy regimes such as radiation therapy, for example.
Huafeng L. et al., in “Robust reconstruction of physiological parameters from dynamic PET data”, 4th IEEE International Symposium on Biomedical Imaging: From Nano to Macro, Apr. 12-15, 2007, 177-180 disclose a model for quantifying physiological and biological processes by means of PET. The state variable inputs of this model are the concentration of the radiotracer in the individual compartments of the tissue such as blood, extracellular space, the volume of the cells and the exchange rate between the compartments. Appropriate measurement models are used to determine the parameters. Flow rates and exchange rates are obtained by appropriate compartmenting of the PET measurement data. The rates of the radiotracer flow from the tissue to the blood, and vice versa, are determined from PET measurements.
DE 10 2005 023 906 A1 discloses a method for determining positron emission measurement information in which a second imaging method for determining perfusion and/or diffusion information is used at the same time as the positron emission tomography generates image records. A contrast agent can also be used for this purpose.
Correctly quantifying the local tracer uptake is important for quantitative evaluation of the pharmacokinetics in the case of dynamic PET studies (tracer kinetic modeling). For this purpose, the acquired raw data has to be corrected in a number of ways, for example for photon scattering in the patient and in the PET detector and for photon absorption. The quantified values for the various tracers are generally specified as so-called standardized uptake values (SUVs). However, the corrections undertaken are limited and are insufficient in particular with regard to the quantification problems described initially.